Free cutting internal diamond grinding wheel

ABSTRACT

Internal diamond grinding wheels typically comprise in combination a nonabrasive metallic core which, in contrast to ordinary grinding wheels, has an outer portion which is adapted to be mounted for rotating and an inner circular perimeter which is the site of grinding. At the inner circular perimeter of the nonabrasive metallic core is an inner abrasive section of diamond abrasive particles which are bonded, typically by electroplating, onto the nonabrasive metallic core. The present invention provides increased useful life for such internal diamond grinding wheels, and an even greater increase in the amount of material which can be cut per carat of diamond, by replacing from about 5 percent to about 75 percent by volume of the diamond abrasive particles with a soft, friable particulate filler, such as olivine.

United States Patent [191' Mackey et al.

[451 Oct. 116, 1973 FREE CUTTING INTERNAL DIAMOND GRINDING WHEEL Inventors: Charles R. Mackey, North Tonawanda; Harold A. Stein, Niagara Falls, both of NY.

Assignee: The Carborundum Company,

Niagara Falls, NY.

Filed: Apr. 6, 1972 Appl. No.: 241,679

[52] U.S. Cl 51/206 R [51] Int. Cl 824d 5/00 [58] Field of Search 51/206 R, 204, 73 R, 51/298, 309

[56] References Cited UNITED STATES PATENTS 3,640,027 2/l972 Weiss 51/206 3,657,845 4/1972 Sekiya 51/20 Primary Examiner-Donald G. Kelly Attorney-David E. Dougherty et al.

[57] ABSTRACT Internal diamond grinding wheels typically comprise in combination a nonabrasive metallic core which, in contrast to ordinary grinding wheels, has an outer portion which is adapted to be mounted for rotating and an inner circular perimeter which is the site of grinding. At the inner circular perimeter of the nonabrasive metallic core is an inner abrasive section of diamond abrasive particles which are bonded, typically by electroplating, onto the nonabrasive metallic core. The present invention provides increased useful life for such internal diamond grinding wheels, and an even greater increase in the amount of material which can be cut per carat of diamond, by replacing from about 5 percent to about 75 percent by volume of the diamond abrasive particles with a soft, friable particulate filler, such as olivine.

8 Claims, 2 Drawing Figures FREE CUTTING INTERNAL DIAMOND GRINDING WHEEL BACKGROUND OF THE INVENTION Internal diamond grinding wheels, comprising a nonabrasive metallic core which is rotated from the outside and contains an inner circular perimeter to which is bonded an inner section comprising diamond abrasive particles and a metallic bond, are commonly used to precision slice hard materials such as silicon ingots or germanium ingots into wafers. These grinding wheels, or ID. (internal diamond) blades as they are sometimes more descriptively called, are made in accordance with standard practice by starting with a thin annular metallic sheet, usually less than 0.25 mm in thickness, and typically only 0.1 mm thick. The inner perimeter of this metallic sheet is electrolytically coated with a layer of diamonds by a metallic bonding material such as nickel. The wheel is mounted in a special grinding machine which holds the outer portion of the nonabrasive metallic core in tension, and rotates the grinding wheel at a high rate of speed. Theworkpiece is then placed inside the inner perimeter of the rotating grinding wheel, and either moved relative to the rotating grinding wheel, or the rotating grinding wheel is moved relative to the workpiece, so that a thin slice can be removed from the workpiece. In order to not waste undue amounts of the workpiece in slicing the silicon or other material into wafers, and to make the cutting operation easier, since the effort expended will be directly proportional to' the material removed in cutting the ingot, the nonabrasive metallic core and the inner abrasive section are made as thin as feasible.

The nonabrasive metallic core is annular in shape, having a circular inner perimeter. The inner abrasive section comprises diamond abrasive particles and a metallic bond, these components being bonded, typically by electroplating onto the circular inner perimeter of the nonabrasive metallic core. Methods of producing such grinding wheels are known in the art (see for example U.S. Pat. No. 2,117,513), and internal grinding wheels made in this fashion are useful in slicing silicon ingots, germanium ingots or the like into wafers which are subsequently cut into smaller pieces for such uses as'the manufacture of electronic components.

Nonetheless, internal diamond grinding wheels in which the innerabrasive section comprises only diamond abrasive particles and metallic bond frequently suffer from premature loss of tension in the nonabrasive metallic core portion of the internal diamond grinding wheel, before all of the abrasive material is worn away. This results in axial movement of the abrasive section, a grinding wheel which is no longer free cutting, imperfect wafers, and the need to replace the internal diamond grinding wheel. This is unfortunate, since the diamond abrasive utilized in these wheels is very expensive.

SUMMARY OF THE INVENTION It has been found that this problem can be overcome by means of an internal diamond grinding wheel, comprising in combination (1) a nonabrasive metallic core having an outer portion adapted to be mounted for rotating and a circular inner perimeter, and (2) an inner abrasive section bonded to the inner circular perimeter of the nonabrasive metallic core, comprising about 50 percent by volume ofa metallic bond and about 50 percomprising from about 5 to about percent by volume of a soft, friable particulate filler, and from about 25 to about 95 percent by volume of diamond abrasive particles.

Nondiamond particulate inclusions added to the bond in effect reduce the diamond concentration which is present, since the particulate content in electroplated bonds is fixed in conventional manufacturing techniques. By the use of a soft, friable particulate filler, fracture and breaking out of the particulate filler occurs during cutting, resulting in a more open, free cutting bond than would otherwise be obtained. High diamond concentrations such as formed in conventional electroplated products are necessary for some cutting off operations. Concentrations higher than that required for a given operation, however, waste diamond and cause a blade to act hard."

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plan view of an internal diamond grinding wheel in accordance with the present invention, comprising a nonabrasive metallic core. 11 and an inner abrasive section 12 bonded to the inner circular perimeter of the nonabrasive metallic core.

FIG. 2 is a greatly expanded partial, sectional view of the internal diamond grinding wheel of FIG. 1, taken along line 22 of FIG. 1, and illustrating the crosssectionof the inner perimeter of the internal diamond grinding wheel. I

DETAILED DESCRIPTION According to the present invention, the inner abrasive section of the internal diamond grinding wheel comprises about 50 percent (plus or minus about l0 percent) by volume ofa metallic bond. This proportion is obtained inherently by the plating process, when an excess of particulate inclusions are included in the plating solution from which the bond is plated onto the nonabrasive metallic core and the particulate inclusions are all of approximately the same size. The particulate inclusions, usually only diamond abrasive particles, comprise the remaining 50 percent (plus or minus about 10 percent) of the inner abrasive section of the internal diamond grinding wheel. In accordance with the present invention, however, the particulate inclusions contained in this inner abrasive section comprise from about 5 percent to about 75 percent by volume of a soft, friable particulate filler, and from about 25 percent to about 95 percent by volume of diamond abrasive particles. Within this range it is preferred that the particulate inclusions within the inner abrasive section comprise fromabout 20 percent to about 50 percent by volume of soft, friable particulate filler, and from about 50 percent to about percent by volume of diamond abrasive particles.

The preferred soft, friable particulate filler for use in the present invention is olivine. Olivine is an abundant rock-forming mineral group consisting essentially of silicates of magnesium and'ferrous iron, i.e., (Mg, Fe) SiO.,. A complete isomorphous series exists, ranging from forsterite, Mg SiO through chrysolite to fayalite, Fe SiO Generally, olivine is richer in magnesium than iron, with fayalite being the minor component and usually averaging about l5 percent of the olivine. Nickel and titanium may be present in minute quantities as impurities. Olivine occurs as orthorhombic crystals and is an essential constituent and characteristic mineral of many basic rocks such as gabbro, basalt, and periodotite. The rock known as dunite is composed almost wholly of olivine. Olivine has a specific gravity ranging from about 3.2 to about 4.3, the specific gravity increasing with increasing iron content. It has a hardness of about 6.5 to 7.0. Olivine contains from about 36 to about 43 percent silica, from about 28 to about 52 percent magnesia, and from about 5 to about 30 percent ferrous iron.

Other soft, friable particulate fillers can also be used, however, such as cured phenolic resin particles. Phenolic resin (i.e., phenol-aldehyde resin) in the condensation product of a phenolic compound, such as phenol or resorcinol, with an aldehyde such as formaldehyde. It is preferred that the filler and diamond abrasive particles be of similar size distributions, i.e., that the maximum particle size, minimum particles size and average particle size for the soft, friable particulate filler be within about percent of the corresponding dimensions of the diamond abrasive particles. It has been found that size 320 grit abrasive grain, and olivine of a similar size distribution, is an effective combination for use in the present invention. 320 grit particles normally have an average size of about 40 microns, a maximum size of about 50 microns and a minimum size of about microns. Smaller or larger diamond abrasive particles and soft, friable particulate filler can of course be used, depending upon the ultimate application for which the internal diamond grinding wheel is to be used.

The nonabrasive metallic core can be of any metal to which the diamond abrasive particles and soft friable particulate filler can be bonded, and hard rolled stainless steel and phosphor bronze are preferred materials for this use. The nonabrasive metallic core should be preferably less than about 0.25 mm thick.

The soft, friable particulate filler should be nonconductive, insoluble in the plating solution, and denser than the plating solution, if the particulate inclusions are bonded by the preferred method, by including. an excess of soft friable particulate filler and diamond abrasive particles in the plating solution from which the metallic bond is plated onto the nonabrasive metallic core. Since the plating solutions generally utilized are aqueous and contain heavy metals in solution, and have a maximum density of about 1.25 gm/cu. cm., the soft, friable particulate filler should be denser than this density, in order that it will not float on the top of the plating solution.

The invention will now be illustrated with an example.

EXAMPLE Referring now to the drawings, a number of internal diamond grinding wheels having an annular nonabrasive metallic core 11 were prepared, each having an outer portion adapted to be mounted for rotating and a circular inner perimeter, to which was bonded an inner abrasive section l2, comprising about 50 percent by volume ofa metallic bond. The nonabrasive metallic core of this example was composed of a phosphor bronze containing about l0 percent tin, less than 1 percent phosphorus, and smaller amounts of other elements, with the balance being copper. This nonabrasive metallic core was approximately 20.6 cm in outside diameter, and had a circular inner perimeter about 7 in diameter. The nonabrasive metallic core was approximately 0.] mm thick.

To nonabrasive metallic core 11 was bonded an internal abrasive section 12, most clearly shown in FIG. 2. The bond used was electroplated nickel, plated from a nickel sulfamate solution. Inner abrasive section 12 extended about 1.5 mm outwards from the circular inner perimeter of nonabrasive metallic core 11, on each side of nonabrasive metallic core 11, an average thickness of about 50 microns; and inward from the circular inner perimeter of nonabrasive metallic core 11 a distance of about 250 microns. The inner portion of the inner abrasive section had an average thickness of about 200 microns, corresponding to the micron thickness of nonabrasive metallic core 11 plus the two thicknesses of the outer portion of inner abrasive section 12, each having a thickness of about 50 microns.

A number of internal diamond grinding wheels were made having such dimensions, some (control" wheels) with an inner abrasive section comprising only about 50 percent by volume diamond abrasive particles and about 50 percent by volume ofa nickel bond which was plated onto the nonabrasive metallic core, and some (invention" wheels) with an inner abrasive section comprising about 50 percent by volume nickel bond, and 50 percent by volume of particulate inclusions comprising 27 percent by volume olivine and 73 percent by volume of diamond abrasive particles. Both the invention wheels and control wheels were manufactured to the same dimensions, and the total volume of particulate inclusions (diamond or diamond and olivine) was the same in both cases. The control wheels therefore contained about 0.244 carat of diamond each, and the invention wheels about 0.149 carat. In all cases the olivine particles and diamond abrasive particles were of 320 mesh size, having an average particle size of about 40 microns, a maximum particle size of about 50 microns, and a minimum particle size of about 25 microns.

The internal diamond grinding wheels thus made were tested on a Hamco grinding machine. The outer portion of the nonabrasive metallic core was rotated so that the inner abrasive section travelled at a constant speed of 1000 surface meters/minute. Silicon rods having a diameter of about 38 mm were sliced into wafers about 250 microns thick. In all cases, the grinding was accomplished with a water lubricant, and an infeed rate of about 19 mm/minute was maintained, so that it took about 2 minutes to cut through the silicon rod ingot. The number of cuts through the silicon rod which were obtained per blade without the internal diamond grinding wheel losing its tension, and therefore producing unacceptable silicon wafers, was recorded. From this data, the average number of silicon wafers per carat of diamond which were produced was calculated. The results are set forth in the following table.

TABLE Average Number of Approximate Number Wheels Cuts Per Wheel of Cuts Per Carat Control 1,690 6,590 Invention 2,401 l6,lO0

From the above, it can be seen that the use of a soft, friable particulate filler in internal diamond grinding wheels increases markedly (in this case, about 42 percent) the number of cuts which can be made with such a grinding wheel without the wheel losing tension, and

even more markedly (in this case, about 130 percent) increases the number of cuts which can be made with such a wheel per carat of diamond used.

We claim:

1. An internal diamond grinding wheel, comprising in combination 1. a nonabrasive metallic core having an outer portion adapted to be mounted for rotating and a circular inner perimeter, and

2. an inner abrasive section bonded to the inner circular perimeter of the nonabrasive metallic core, comprising about 50 percent by volume of a metallic bond and about 50 percent by volume of particulate inclusions, said inclusions comprising from about 5 to about 75 percent by volume of olivine and from about 25 to about 95 percent by volume of diamond abrasive particles.

2. The internal diamond grinding wheel of claim 1, wherein said particulate inclusions comprise from about 20 percent to about 50 percent by volume of soft friable particulate filler and from about 50 percent to about 80 percent by volume of diamond abrasive particles.

3. The internal diamond grinding wheel of claim 1, wherein the soft, friable particulate filler is cured phenolic resin particles.

4. The internal diamond grinding wheel of claim 1, wherein the soft, friable particulate filler and the diamond abrasive particles are of similar size.

5. The internal diamond grinding wheel of claim 1, wherein the soft, friable particulate filler and the diamond abrasive particles range in size from about to about 50 microns.

6. The internal diamond grinding wheel of claim 1, wherein the nonabrasive metallic core is phosphor bronze.

7. The internal diamond grinding wheel of claim 1, wherein the nonabrasive metallic core is hard rolled stainless steel.

8. The internal diamond grinding wheel of claim 1, wherein the nonabrasive metallic core is less than about 0.25 mm thick. 

2. The internal diAmond grinding wheel of claim 1, wherein said particulate inclusions comprise from about 20 percent to about 50 percent by volume of soft friable particulate filler and from about 50 percent to about 80 percent by volume of diamond abrasive particles.
 2. an inner abrasive section bonded to the inner circular perimeter of the nonabrasive metallic core, comprising about 50 percent by volume of a metallic bond and about 50 percent by volume of particulate inclusions, said inclusions comprising from about 5 to about 75 percent by volume of olivine and from about 25 to about 95 percent by volume of diamond abrasive particles.
 3. The internal diamond grinding wheel of claim 1, wherein the soft, friable particulate filler is cured phenolic resin particles.
 4. The internal diamond grinding wheel of claim 1, wherein the soft, friable particulate filler and the diamond abrasive particles are of similar size.
 5. The internal diamond grinding wheel of claim 1, wherein the soft, friable particulate filler and the diamond abrasive particles range in size from about 25 to about 50 microns.
 6. The internal diamond grinding wheel of claim 1, wherein the nonabrasive metallic core is phosphor bronze.
 7. The internal diamond grinding wheel of claim 1, wherein the nonabrasive metallic core is hard rolled stainless steel.
 8. The internal diamond grinding wheel of claim 1, wherein the nonabrasive metallic core is less than about 0.25 mm thick. 